In a typical cellular radio communications system (wireless telecommunications network), an area is divided geographically into a number of transmission areas, or cell sites. Each cell site is typically defined by a radio frequency (RF) radiation pattern from a base transceiver station (BTS) antenna. The base station antennae in the cells may then be coupled to a base station controller (BSC) (also known as a radio network controller (RNC)), which may in turn be coupled with a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN). And the switch or gateway may then provide connectivity with a transport network such as the Public Switched Telephone Network (PSTN) or the Internet.
When a wireless communication device (WCD), such as a cellular telephone, pager, personal digital assistant (PDA), or appropriately equipped portable computer, is positioned in a cell, the WCD communicates via an RF air interface with the BTS antenna of the cell. Thus, a communication path can be established between the WCD and the transport network, via the RF interface, the BTS, the BSC and the switch or gateway.
With the rapid growth in demand for wireless communications, the level of call traffic in most cell sites has increased dramatically in recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined by radiation patterns from directional antenna components of the BTS, or by BTS antennae. These sectors (which can be visualized ideally as pie pieces) may be referred to as “physical sectors,” since they are physical transmission/reception areas (hereafter “transmission area(s)”) of a cell site. Therefore, at any given time, an MS in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the telecommunications network via the BTS serving that physical sector.
In well known CDMA (Code Division Multiple Access) systems, each physical sector is distinguished from geographically adjacent physical sectors by a “PN offset” number or key. PN offsets are pseudo-noise (e.g., deterministic “noise-like” information) that is inserted in the carrier signal for the corresponding sector. When a WCD is in a particular physical sector, communication between the WCD and the BTS of the cell site are encoded by the physical sector's PN offset key, regardless of the carrier frequency being used. This allows the same carrier frequency to be used by geographically adjacent sectors with minimal interference between the sectors occurring.
In areas where wireless communication traffic is particularly high, cell sites in those areas may employ more than one carrier frequency for communicating with the WCDs that are within its transmission/reception area boundaries. The number of carrier frequencies employed by a given cell site may depend on various factors, such as the volume of communication traffic expected. For example, in a congested urban location, cell sites might be designed to employ two or more carrier frequencies, while in more sparsely populated rural areas, cell sites might employ only one carrier frequency.
Cell sites that employ more than one carrier frequency may be termed as having a “primary” carrier frequency and one or more “overlay carrier frequencies.” Typically, the primary frequency is the carrier frequency that is implemented by all the cell sites in a particular geography, such as in a particular city and its surrounding area, and is often labeled frequency “F1.” Overlay frequencies are then implemented by the cell sites in the geography that carry more traffic than may be handled using only the primary frequency, and are typically labeled “F2”, “F3”, etc.
In normal operation, when a WCD is operating on a given frequency and moves into a sector of a cell site that is operating on the same frequency, the call will typically continue on that same frequency in the new sector (e.g., if the new physical sector is controlled by the same BSC or both physical sectors are otherwise coupled with common infrastructure). Through communication with the BSC, the WCD may simply begin to use the PN offset key of the new physical sector in order to complete the handoff from one physical sector to the next. This process is called “soft handoff,” because, as the call continues on the same carrier frequency, it typically maintains communication on the old PN offset for at least some time after beginning to also communicate on the new PN offset. Such handoff thus tends to be fairly seamless and successful.
In contrast, when a WCD is operating on an overlay frequency carrier signal and the WCD moves into (or toward) a cell site that does not implement that particular overlay carrier frequency (“a hand-down cell site”), a process known as a “hard handoff” may need to occur in order to maintain the call. In a hard handoff, the call is moved from a carrier signal of the overlay frequency to a carrier signal of the primary frequency or to another overlay frequency. Unfortunately, however, since hard handoff involves a change in carrier frequency, communication on the old PN offset and old carrier frequency is typically discontinued before the WCD begins communicating on the new PN offset and new carrier frequency. This process is less seamless and tends to result in more dropped calls than soft handoff.
Consequently, although it may be desirable for a service provider to add overlay carriers in certain high traffic areas, the addition of such carriers may give rise to problems. In particular, addition of overlay carriers may create carrier-discontinuities across cell sites and thus increasing the likelihood of hard handoffs and, in turn, call drops.